body{
    margin: 0;  /*外边距置位0*/
    padding: 0;
    text-decoration: none;
}

#app{
    width: 300px;
    border: 1px solid red;
}

h2{
    text-align: center;
    background: brown;
    margin: 0;
}

form{
    background: aqua;
}

div:nth-of-type(1)>input{
    border: 3px solid black;  /*solid 实线    dashed 虚线*/
}
div:nth-of-type(2)>input{
    border:3px dashed red;
}
div:nth-of-type(3)>input{
    border: 3px solid rgba(54, 8, 8, 0.79);
}
